Yes the RX 470 would perform to it's fullest in your system, but yes you need at least another 4GB of system memory. Might as well get the same exact RAM and run it in dual-channel.
A decent 450-500W power supply should be sufficient. It really depends on your budget.
If on a tight budget something like
this.
If you want a higher quality unit, then
this.
And if you have plenty to spend, then something like
this.